The mineralization of calcium carbonate (CaCO(3)) in the brown seaweed Padina gymnospora is a biologically induced process and is restricted to the cell wall surface. It has been suggested that the CaCO3 crystallization that occurs over the thallus cell wall surface is induced by changes in the surface pH caused by a local efflux of OH(-), Ce(++) and HCO(3)(-) ions. However, no studies on the roles of the P. gymnospora cell wall components in this mineralization process had been performed. Therefore, we evaluated the influence of a subset of P. gymnospora cell wall molecules on CaCO3 crystallization in vitro. The molecules tested were the anionic polysaccharides alginates and fucans (with potential nucleation activity) and phenolic substances (secondary metabolites with amphipathic property). The crystallization assays were performed using polystyrene microbridges as the crystallization apparatus. Crystals formed in the microbridges were analyzed using scanning electron microscopy. Interestingly, the results confirmed that the phenolic substances have the specific capability of changing the morphology of calcite crystals grown in vitro by inducing an elongated morphology in the direction of the c-axis. This morphology is similar to that induced by molecules that attach to {h k 0}-crystal planes. It was also shown that the alginates and the fucans do not specifically modulate the morphology of the growing crystals. In fact, these crystals exhibited a rounded shape due to the slower growth rates of several new crystal planes that appeared in the place of the original corners and edges. (C) 2011 Elsevier B.V. All rights reserved.